On Mon, Feb 17, 2014 at 01:26:49PM -0800, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> > > The ACCESS_ONCE() primitive provides cache coherence, but the > documentation does not clearly state this. This commit therefore upgrades > the documentation. > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Punctuation nit below; otherwise: Reviewed-by: Josh Triplett <j...@joshtriplett.org> > Documentation/memory-barriers.txt | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/Documentation/memory-barriers.txt > b/Documentation/memory-barriers.txt > index 102dc19c4119..ad6db1d48f1f 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -1249,6 +1249,23 @@ The ACCESS_ONCE() function can prevent any number of > optimizations that, > while perfectly safe in single-threaded code, can be fatal in concurrent > code. Here are some examples of these sorts of optimizations: > > + (*) The compiler is within its rights to reorder loads and stores > + to the same variable, and in some cases, the CPU is within its > + rights to reorder loads to the same variable. This means that > + the following code: > + > + a[0] = x; > + a[1] = x; > + > + Might result in an older value of x stored in a[1] than in a[0]. > + Prevent both the compiler and the CPU from doing this as follows: > + > + a[0] = ACCESS_ONCE(x); > + a[1] = ACCESS_ONCE(x); > + > + In short, ACCESS_ONCE() provides "cache coherence" for accesses from > + multiple CPUs to a single variable. You don't need to "quote" the well-established term "cache coherence". > (*) The compiler is within its rights to merge successive loads from > the same variable. Such merging can cause the compiler to "optimize" > the following code: > -- > 1.8.1.5 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
